The present invention relates in general to a ceramic envelope device for use in a high-pressure discharge lamp (hereinafter referred to as "HID lamp"; "HID" representing High Intensity Discharge), and more particularly to electrically conductive end caps or closure discs which close the opposite ends of a translucent ceramic tube which cooperates with the end caps to form a gas-tight envelope incorporated in a HID lamp.
In the art of such HID lamps using a translucent ceramic tube, a pair of electrically conducting discs are known as end caps to close the opposite open ends of the translucent ceramic tube. Examples of such closure end caps are illustrated in U.S. Pat. Nos. 4,155,757 and 4,155,758. Such end caps are formed of an electrically conducting cermet obtained by mixing, for example, particles of tungsten with particles of aluminum oxide, and sintering the mixture. These electrically conducting cermet end caps support a pair of tungsten electrodes at their inner surfaces of the ceramic envelope device so that the electrodes protrude from the inner surfaces of the end caps toward each other, i.e., longitudinally inwardly in the translucent ceramic tube. Also, electrical contact rods or lead rods are connected or fixed to the outer surfaces of the cermet end caps by suitable methods, so that electric power is applied to the pair of opposed tungsten electrodes through the contact rods and through the cermet end caps. Such cermet end caps have been advantageously employed, for example, in high-pressure sodium lamps, because they eliminate the need of using expensive metallic niobium. It is further recognized that such cermet end caps have been used also advantageously for so-called metal halide lamps which employ translucent ceramic tubes charged with a suitable metal halide together with mercury and rare gas, because the cermet exhibits relatively high corrosion resistance to metal halides.
However, such a HID lamp with a translucent ceramic tube closed by cermet end caps may suffer from a problem generally known as "arc-back" phenomenon wherein an arc will take place between the electrodes and the corresponding cermet end caps, rather than between the opposed electrodes, when the HID lamp is initially turned on. This arc-back phenomenon causes the cermet end caps to crack, thereby causing the translucent ceramic tube to leak. In addition, the "arc-back" phenomenon gives rise to vaporization and the scattering of refractory metal components in the cermet, and consequent deposition thereof on the inner surfaces of the ceramic tube, which results in blackening of the wall of the ceramic tube, thereby reducing its luminous flux.
It is also recognized that supersaturated metal halide in the ceramic tube may condense at the cold spot in the tube, i.e., at the lower end portion of the ceramic tube disposed vertically when the lamp is used in its upright position, whereby the cermet end caps closing the end portions of the tube are subject to corrosion due to the liquid phase of the condensed metal halide, with a result of failing to stably support the electrode in its upright posture, if the corrosion becomes severe.